use tc as default engine in Docker

CHANGELOG.md

@@ -2,6 +2,7 @@
 ## UNRELEASED
 ### Fixed
 * Cli help options for Browsertime was very unclear and unspecific. 
+* TSProxy is somehow broken together with Selenium. TC is now default connectivity engine when running in Docker.
 
 ### Added
 * Made the size table sorteable for PageXray metrics

Dockerfile

@@ -1,6 +1,7 @@
 FROM sitespeedio/webbrowsers:firefox-50.0-chrome-54.0
 
 ENV SITESPEED_IO_BROWSERTIME__XVFB true
+ENV SITESPEED_IO_BROWSERTIME__CONNECTIVITY__ENGINE tc
 
 RUN mkdir -p /usr/src/app
 WORKDIR /usr/src/app